1. Field of the Invention
The present invention relates to a clock signal extraction system and, more particularly, to a clock signal extraction system that extracts a clock signal from a received optical signal. The present invention also relates to a method of extracting a signal therefor.
2. Description of the Background Art
The clock signal extraction system is based on a technology that extracts from a received optical signal a clock signal providing the temporal reference of the optical signal and uses the extracted clock signal for gating to obtain information from the received optical signal.
Japanese Patent Laid-open Publication No. 2006-49967 discloses a trial clock signal extraction system including a hybrid component that may extract electrically and optically a clock signal from an optical signal. The clock signal extraction has thus been developed aiming at improving the performance of the extraction system.
A clock signal extraction system in a receiver of the optical time division multiplexing communication system is disclosed in Hiromi Tsuji et al., “Development of 160 Gb/s Clock Extraction Using Electro-Absorption Modulator,” Proceedings of 2003 Conference, Institute of Electronics, Information and Communication Engineers (IEICE), B-10-115, 2003.
The conventional clock signal extraction systems disclosed use a bit rate of 40 [Gbit/s] per channel, and allow an optical signal on four channels to be optically time division multiplexed. The transmitter system in the optical time division multiplexing communications system is based upon the conventional clock signal extraction system, and transmits a return-to-zero (RZ) encoded optical pulse signal of 160 [Gbit/s] by the four-channel time-division multiplexing. The clock signal extraction system in the receiver therefore extracts a clock signal of 40 [GHz] from the optical pulse signal of 160 [Gbit/s].
When received by the receiver, an input optical signal is input to a clock signal extraction system. The input optical signal is of an RZ encoded optical pulse. The clock signal extraction system includes an electro-absorption modulator (EAM), an opto-electric converter, a first band pass filter, a phase comparator, a quadruple multiplier, a clock signal generator, a loop filter, a voltage controlled oscillator (VCO), a mixer, a second band pass filter, and an amplifier.
In the clock signal extraction system, the input optical signal is first input to the EAM modulator. The EAM modulator also receives as a control signal a modulation electrical signal. The modulation electrical signal is generated as follows. The VCO oscillator generates an electrical signal at a frequency of 40 [GHz]. The clock signal generator generates a reference electrical signal at a frequency f0 in the order of Gigahertz (GHz). The mixer mixes the electrical signal at a frequency of 40 [GHz] with the reference electrical signal. The mixed signal is filtered by the second band pass filter and amplified by the amplifier, thus providing the modulation electrical signal. The second band pass filter has its transmission band of which the center frequency is equal to (40-f0) [GHz].
When the optical signal propagates over the optical waveguide in the EAM modulator, the absorption coefficient of the optical waveguide is dependent upon the frequency of the modulation electrical signal input to the EAM modulator. Specifically, the optical signal propagating over the optical waveguide is transmitted or shut off at a frequency of (40-f0) [GHz]. The optical pulse signal of 160 [Gbit/s] is input to the EAM modulator. The EAM modulator outputs a component of the input optical signal that passes through a transmission window of (40-f0) [GHz] in the form of modulated optical pulse signal. The modulated optical pulse signal is input to the opto-electric converter where it is photoelectrically converted and output as a first electrical signal to the first band pass filter.
The first band pass filter has its transmission band whose center frequency is equal to 4×f0 [GHz]. The first filter filters the first electrical signal. The first band pass filter thus outputs a second electrical signal at a frequency of 4×f0 [GHz]. The second electrical signal is input to the phase comparator. The phase comparator compares in phase the input second electrical signal with a third electrical signal from the clock signal generator. The third electrical signal corresponds to a signal output from a quadruple multiplier when received the reference electrical signal at a frequency f0 in the order of GHz input via a splitter. The phase comparator outputs, when the second electrical signal and the third electrical signal are in phase with each other, a fourth electrical signal of 0 V. The phase comparator also outputs, when the second electrical signal and the third electrical signal are out of phase from each other, a fourth electrical signal having a voltage proportional to the phase difference.
The fourth electrical signal is input to the loop filter. The loop filter outputs a signal representative of intensity averaged over time as a fifth electrical signal to the VCO oscillator. The VCO oscillator has a function of outputting a sixth electrical signal at a frequency proportional to the voltage of the fifth electrical signal. The frequency of the sixth electrical signal from the VCO oscillator thus changes so that the second and third electrical signals are in phase with each other. The VCO oscillator is set to receive the fifth electrical signal of 0 V and output a signal at a frequency of 40 [GHz]. Therefore, when the second and third electrical signals are in phase with each other, the VCO oscillator outputs the sixth electrical signal at a frequency of 40 [GHz]. Specifically, in order to provide the sixth electrical signal at a frequency of 40 [GHz], the second electrical signal and the third electrical signal need to be synchronized with each other.
The modulated optical pulse signal is a signal which the EAM modulator performs modulation by a frequency resultant from mixing a frequency (40 [GHz]) equal to one fourth of the clock frequency (160 [GHz]) of an input optical pulse signal with a low frequency component f0 in the order of GHz to output. The first electrical signal is a signal into which the modulated optical pulse signal is converted by the opto-electric converter. The second electrical signal is the frequency component of 4×f0 [GHz] filtered from the frequency components of the first electrical signal that is transmitted by the first band pass filter. Therefore, the fact that the third electrical signal and the second electrical signal are in phase with each other corresponds to the fact that the input optical signal and the reference electrical signal are in phase with each other.
The sixth electrical signal output from the VCO oscillator is split by a splitter into two signals, one of which is input to the mixer.
The mixer receives the sixth electrical signal from the VCO oscillator and an electrical signal at a frequency f0 from the clock signal generator via the splitter. The mixer then outputs a seventh electrical signal resultant from combining a plurality of frequency components of (40±n×f0) [GHz]. Of the frequency components of the seventh electrical signal, only the electrical signal having a frequency component of (40-f0) [GHz] is transferred through the second band pass filter to the amplifier. The sixth electrical signal is split by the splitter into two signals as described above, the other of which is output from the clock signal extraction system as an extracted clock signal.
For an optical pulse signal involving a large timing jitter, Japanese '967 Publication proposes a method and a system that may extract a clock signal with the timing jitter removed.
Japanese '967 Publication indicated above discloses a different clock signal extraction system. The clock signal extraction system disclosed by the Japanese publication differs from the above clock signal extraction system in that it includes a first EAM modulator, an optical amplifier, and a second EAM modulator, all of which are cascade-connected in this order. The first EAM modulator modulates an optical pulse signal. The optical amplifier amplifies a first modulated optical pulse signal from the first EAM modulator. The second EAM modulator modulates the output signal from the optical amplifier, outputting a second modulated optical pulse signal. The electrical signal from the amplifier is split into two electrical signals. One of the electrical signals is input to the first EAM modulator, and the other electrical signal is phase-adjusted by the phase adjuster and then input to the second EAM modulator.
The input optical signal to the clock signal extraction system thus passes through the two EAM modulators. That makes it possible to narrow the duration of the second modulated optical pulse signal.
In the above two examples, the clock signal extraction system extracts the clock signal from the RZ encoded optical pulse signal. In contrast, another Japanese Patent Laid-open Publication No. 2005-252942 proposes a system and a method that extract a clock signal from a non-return-to-zero (NRZ) encoded optical pulse signal. The clock signal extraction system includes an optical phase controller, an opto-electric converter, and a band pass filter.
The optical phase controller receives an NRZ encoded input optical signal. The optical phase controller converts the input optical signal into the RZ encoded optical pulse signal. The opto-electric converter converts the optical pulse signal into an electrical pulse signal. The band pass filter extracts a clock signal component from the electrical pulse signal, and outputs it as an electrical clock signal.
The optical phase controller includes an optical splitter, an optical phase adjuster, and an optical multiplexer. The optical splitter splits the input optical signal encoded using the NRZ code into a first and a second NRZ signal. The optical phase adjuster delays the phase of the first NRZ signal. The optical multiplexer multiplexes the first NRZ signal that is phase-delayed by the optical phase adjuster and the second NRZ signal, thus generating a resultant optical pulse signal.
The phase delay of the first NRZ signal in the optical phase adjuster is set to a half (T/2) of the period (T) of the input optical signal. The optical phase adjuster is set so that the first NRZ signal is off-set in phase of the carrier wave by exactly half the wavelength against the second NRZ signal.
The input optical signal encoded using the NRZ code may thus be converted into the RZ encoded optical pulse signal, extracting a clock signal of frequency equal to the bit rate.
Another Japanese Patent Laid-open Publication No. 341239/1998 discloses a multi beat serial ATM line receiver. The ATM receiver is adapted to be able to automatically switch between two types of subscriber packages for 155 Mbps and 622 Mbps as if they were of a single subscriber package. The ATM receiver includes two systems of circuitry for 155 Mbps and 622 Mbps each comprised of a clock extraction circuit, a data retiming circuit and a header error control (HEC) detection circuit, and a single 155 Mbps/622 Mbps determination circuit. The clock extraction circuits each extract a clock signal from the received serial data. The data retiming circuits each perform data retiming. The HEC detection circuits each extract HEC data for detecting and correcting header error in a cell header. The 155 Mbps/622 Mbps determination circuit instructs a selector to select and send the received serial data that continuously detects the HEC data to the subsequent stage circuit.
Propagation of optical signals is possible under the condition that the more data flows over the network the more optical signals having different bit rates may propagate.
The above conventional clock signal extraction systems are used in a network having a fixed bit rate, and may extract a clock signal at a constant frequency. When data at different bit rates are propagated as in the giga bit Ethernet (trade name) and the SDH/SONET (Synchronous Digital Hierarchy/Synchronous Optical Network), for example, however, respective clock signal extraction systems are required corresponding to those bit rates. Even when data has the same bit rate, data added with a forward error correction code and data not added with the code have a substantially extended bit rate, so that two clock signal extraction systems are required respectively corresponding to data with the forward error correction code and data not with the code.